DESCRIPTION: The murine model of Leishmania major infection is an exceptional model for investigations of helper T cells subset differentiation in vivo. Decisions that ultimately drive development of Tbl or Tb2 cells underlie the outcome of the disease, and the availability of inbred strains of mice differing in outcome offers unique opportunities to model factors involved in this complex process. Studies to date have revealed oligoclonal expansion of helper T cells in susceptible mice that arise in response to a single parasite determinant from the LACK antigen. The hypothesis that T cell recognition of such precision might underlie disease susceptibility remains unprecedented, but still must be rigorously established. This proposal will take two strategies that seek to test unequivocally the LACK hypothesis. In the first strategy, we propose to replace the LACK genes in L. major with mutated LACK containing altered peptide ligands in place of the immunodon-dominant epitope for the I- Ad class U N4HC molecule. The hypothesis predicts that such parasites will be controlled in I-Ad-expressing BALB/c mice, but will retain unchanged virulence for BALB.B and BALB/K mice. In the second strategy, we propose to characterize the size, phenotype, effector capacity and turnover of the innate LACK-reactive repertoire in both resistant and susceptible strains of mice congenic at the MHC loci. The hypothesis predicts that innate characteristics of the LACK-reactive repertoire might underlie the divergent responses of these cells on differing genetic backgrounds. In the final strategy, we propose to combine results of the first two approaches to reconstruct the developing immune response in vivo, allowing an in situ visualization of T helper subset differentiation in a complex biologic infectious process. Together, the proposed approaches will bring exceptional clarification to a process - the acquisition of effector function by naive helper T cells - that underlies essentially all immune responses. The use of a parasite model in which the outcome has been so clearly tied to this decision offers an opportunity for important insights into disease susceptibility and effective vaccine strategy.